1. Technical Field
The present invention relates to a projector, a projection unit and an interactive board, and more particularly, to a projector for ultra-short projection.
2. Related Art
In recent years, projectors including a projection optical system for so-called ultra-short projection to allow display of a large-sized screen over a short projection distance have been manufactured. The use of such a projection optical system allows display of a large-sized screen over an extremely short distance (for example, a 100-inch screen over a projection distance of 40 cm) as compared to a front projection type projector in the related art. Generally such projectors can be used only for extremely short distance projection, but it is expected that the capabilities of such projectors, which are well known in the art, can be extended from extremely short distance projection to middle and long distance projection, depending on their usage purposes. For example, JP-A-2002-6398 discloses a projector technique for expanding a zoom ratio of a projection lens by means of a reflecting projection unit mounted at a screen side. In this technique, it is possible to obtain images having different zoom ratios under a state where the reflecting projection unit is attached and a state where it is detached.
If the reflecting projection unit of JP-A-2002-6398 is to be applied to extremely short distance projection, there is a need to significantly increase the zoom ratio. In this case, it is very difficult to provide a decreased aberration with the increased zoom ratio. In addition, with the arrangement where a plurality of curved surface mirrors are eccentric to an optical axis (center axis) of lenses, it is likely that even a small deviation of any optical element may have a great effect on images. Accordingly, in order to obtain a desired optical performance, there is a need for adjustment with very high precision and further it is difficult to decrease an aberration using an eccentric optical system.
With the recent increase in multimedia content, so-called interactive boards have been popularized and used in the fields of education, presentation and the like. An interactive board is characterized in that, while content is displayed, a user can write something to the content. Typically, since the interactive board is the same size as existing blackboards or whiteboards, the interactive board requires a relatively extensive range display. However, application of a typical direct-view type monitor for the extensive range display of the interactive board may cause problems in terms of weight, power consumption and cost of the entire apparatus.
For example, JP-A-2003-136892 and JP-A-2004-252345 disclose an interactive board technique for reflecting image light, which is projected from a projector, to a transmission type screen by a plane mirror. JP-A-2009-83277 discloses a technique of an interactive board including an extremely short projection distance type projector. An enlarged projection using a projector allows extensive range display. The use of a projector allows reduction of weight, power consumption and cost of an apparatus. However, in the techniques disclosed in JP-A-2003-3.36892 and JP-A-2004-252345, the extensive plane mirror is placed to be inclined to the transmission type screen, thereby increasing a size in a depth direction, which may result in poor installability. The projector in the technique disclosed in JP-A-2009-83277 can be only applied to extremely short distance projection, which results in limited usage and poor convenience.